Experiments on the characteristics of saturated boiling heat transfer in a plate heat exchanger for ammonia/lithium nitrate and ammonia/(lithium nitrate+water)

摘要

The new developments in absorption systems for air-conditioning applications respond to the new energetic and environmental situation that requires systems more efficient, use of renewable energy sources, such as thermal solar energy, and integration in polygeneration systems for the energy supply of buildings. Among these new developments, it''s of great interest to mention absorption systems designed specifically for solar applications and direct heat recovery from exhaust gases of micro-generation systems. Some of these new systems are already air-cooled. Ammonia-water mixture is one of the most used working pairs in absorption refrigeration systems; nevertheless, several authors have proposed the use of lithium nitrate as an absorbent instead of water. Ammonia/lithium nitrate refrigeration systems do not require a rectifier to remove the absorbent from the vapor stream leaving the desorber, and the driving temperature is lower than that required for ammonia-water systems. The main disadvantage of this working fluid is related with its high viscosity, which penalizes heat and mass transfer processes in the absorber and generator. To overcome this limitation a double solution is proposed. First, the use of plate heat exchangers in the design of the main components of the cycle and, second, the addition of a small amount of water to the ammonia/lithium nitrate mixture in order to reduce the viscosity but avoiding the need of rectification. In this work, we experimentally investigated saturated flow boiling heat transfer of the ammonia/lithium nitrate and ammonia/ (lithium nitrate + water) mixtures with water content in the absorbent of 20 % by weight, flowing in a vertical plate heat exchanger. The test section consists of four commercial plates with a chevron angle of 60°, referred to the vertical axis of the plate, forming three channels. The effects of heat flux ranging from 5 to 20 kW/m², mass fluxes from 50 to 100 kg/s.m² --and mean vapor quality from 0 to 0.2 on the boiling coefficient and total pressure drop were analyzed. The results show that, at the considered operating conditions, boiling heat transfer coefficient increases with increasing the heat flux and mass flux, whereas the vapor quality slightly influences the boiling heat transfer coefficient. The addition of water increases the boiling heat transfer coefficient more than 30 % when compared with the binary mixture.